1. Field of the Invention
The present invention relates to an external cavity laser capable of oscillating a laser beam having a given oscillation wavelength by means of an optical fiber having the Bragg wavelength of light reflected by a grating adjusted to a given wavelength, i.e., a fiber Bragg grating (hereinafter referred to as xe2x80x9cFBGxe2x80x9d).
2. Description of the Related Art
Conventionally, some lasers of this type, such as the one described in U.S. Pat. No. 4,786,132, oscillate a single-wavelength laser beam with the use of an FBG in an external cavity. One such laser appears in OECC ""96 (First Optoelectronics and Communications Conference Technical Digest, July 1996, Makuhari Messe), 18P-18. This laser, using the FBG in its external cavity, has a so-called lensed-fiber arrangement such that an end facet of a fiber, which is an optical junction lensed.
According to this laser, the transmission quality of transmitted signals is evaluated by the signal-to-noise ratio characteristic. In the case of picture transmission, for example, the relative intensity of noise (RIN) is adjusted to xe2x88x92130 dB/Hz or less. Thereupon, the inventors hereof conducted an experiment in which signals were transmitted under the following conditions using an apparatus model that is constructed in the same manner as the prior art example described in OECC ""96, as shown in FIG. 5.
This apparatus is a laser that has a strained multi quantum well structure, for example. The laser comprises a laser light emitting device 10 formed of a laser diode, for use as a light source, and an FBG section 20, which is a narrow-band reflector-type optical fiber functioning as an optical waveguide and having its reflection-peak wavelength adjusted to the Bragg wavelength. In this arrangement, the laser light emitting device 10 includes an active layer (not shown) and antireflection and high-reflection surfaces 11 and 12 formed on either side of the active layer. On the other hand, the FBG section 20 includes a lensed fiber having a first end facet 21 lensed in the shape of a hemispherical surface, a grating 22 formed in the fiber core, and a second end facet 23 that is equipped with a connector 30. In the laser constructed in this manner, light is generated in the active layer by injected current, and it is reflected by an external cavity, which is formed between the high reflection surface 12 and the grating 22, and is delivered as a laser beam from the second end facet 23 through the connector 30.
Parameters for the laser with this arrangement were set as follows. In the laser light emitting device 10, the field reflectance of the antireflection surface 11 was set at 10xe2x88x924 or less, and the length from the antireflection surface 11 to the high-reflection surface 12 was adjusted to 600 xcexcm or less. In the FBG section 20, the field reflectance and the full width at half maximum for the Bragg wavelength were set at 0.4 or less and 0.1 mm, respectively. The first end facet 21 was subjected to antireflection coating, its field reflectance was set at 0.4 or less, and the optical coupling efficiency was adjusted to 0.5.
FIG. 6 is a characteristic diagram showing a noise characteristic obtained from the experiment. Based on this result, the inventors hereof confirmed that the level of noise produced by connector connection, that is, the intensity level of reflected light that returns from the connector back to the laser, would inevitably influence the transmission band. Thereupon, the inventors hereof have measured the relative intensity noise (RIN) for the case where a physical connector (PC), superphysical connector (SPC), angled physical connector (APC) were connected individually to the second end facet 23 of the optical fiber. FIG. 7 is a diagram showing the results. For the case xe2x80x9cNO ISOLATORxe2x80x9d shown in FIG. 7, RIN exceeded xe2x88x92130 dB/Hz irrespective of the connector type. Carrying out picture transmission in this state would result in lowered quality of picture transmission, with significant noise appearing on the screen.
The present invention has been contrived in consideration of these circumstances, and its object is to provide an external cavity laser capable of obtaining satisfactory transmission quality at all times irrespective of a connector or connectors to be connected.
In order to achieve the above object, in an external cavity laser according to the present invention, an FBG section is formed having the Bragg wavelength of light reflected by a grating adjusted to a given wavelength. A laser light emitting device having a reflective surface, which generates light, is optically coupled to the FBG section to ensure input and output of light. The generated light is reflected by the reflective surface. A cavity is formed including the reflective surface of the laser light emitting device and the grating. The cavity resonates the light between the reflective surface and the grating, thereby oscillating a laser beam having a given oscillation wavelength through the connector. Further, intercepting means, such as an isolator, is located on an optical path between the cavity and the connector, and intercepts reflected waves from the connector.
Thus, according to the invention, the isolator on the optical path between the cavity and the connector absorbs and intercepts the reflected waves or reflected return light from the connector. Accordingly, the noise characteristic is improved so that the noise level has no influence upon signals in the transmission band irrespective of the connector to be connected. In consequence, satisfactory transmission quality can be obtained at all times.
In the case where the FBG section of the external cavity laser is located on an optical path between the laser light emitting device and the connector, the isolator is located on an optical path between the FBG section and the connector so that the reflected waves from the connector can be absorbed and intercepted by means of the isolator to improve the noise characteristic.
In the case where the FBG section of the external cavity laser is located on an optical path situated on the opposite side of the laser light emitting device from the connector, the isolator is located on an optical path between the laser light emitting device and the connector so that the reflected waves form the connector can be absorbed and intercepted by means of the isolator to improve the noise characteristic.